1. Technical Field
This invention relates generally to wind turbines and the like, and more particularly to a vertical axis wind turbine that utilizes the kinetic energy of moving air to provide rotational energy useable for generating electric power and/or other useful purposes.
2. Description of Related Art
The goal of any wind turbine is to extract power from the wind. In 1919 Albert Betz theorized that the efficiency limit for wind energy conversion is 59.3% and emphasized that since energy is derived from the movement of air molecules, one should avoid slowing the incoming air mass by more than about one-third. Based on the work of Betz, wind turbine technology has evolved toward the use of a minimum number of airfoils traveling at a maximum practical speed, in an attempt to create a virtual two-dimensional surface area through which the passing wind energy can be extracted.
An existing wind turbine usually takes the form of a horizontal axis wind turbine (HAWT) or a vertical axis wind turbine (VAWT). Due largely to Betz, wind energy research has tended to focus on HAWTs because issues pertaining to efficiency, such as blade-tip speed and airfoil design, are more relevant to propeller-type HAWT designs than to VAWT designs, which are sometimes viewed as little more than novelties. Many VAWT designs today still operate in drag mode, deriving torque by exploiting drag in the downwind phase of the cycle while attempting to minimize drag on the upwind leg, an approach that can be traced back in time to caged-rotor VAWTs and other such panemones.
Attempts to modernize VAWTs by incorporating lift-thrust have produced a class of machines called giromills and cycloturbines (i.e., VAWTs operating in lift mode). VAWTs operating in lift mode fall into three major subcategories. The first subcategory, fixed-pitch VAWTs, includes those having fixed airfoils (e.g., the Darrieus machine described in U.S. Pat. No. 1,835,018). The second, self-orienting-pitch VAWTs, includes those using reactive elements that orient themselves relative to the wind without a separate control means (less-than-ideal angle-of-attack due to the delay inherent in repositioning the airfoil reactive elements in response to the same relative wind from which they derive their motive lift). The third, mechanically-controlled-pitch VAWTs, are those that utilize variable pitch controlled by mechanical methods. There are also aspects of variable geometry in each subcategory, used for both performance considerations and over speed and high wind protection, sometimes referred to as furling.
With further regard to the Darrieus patent mentioned above (U.S. Pat. No. 1,835,018), it describes a method of cyclical pitch control of airfoils using a shifting central rotor post, and then proceeds to demonstrate that mechanical pitch variation can be abandoned if properly shaped airfoils are accelerated to velocities well above local wind speeds. Doing so creates relative winds that continuously impinge favorably on the airfoil surfaces, creating lift. The cost of this simplification, however, is the loss of a self-starting capability. Nevertheless, the Darrieus patent and its theoretical basis have formed an important foundation for much of VAWT technology and research that followed.
The VAWT described in U.S. Pat. No. 4,299,537 (i.e., the Evans patent) also sets the cyclical pitch angles of the airfoils without any external orienting means. It does so by allowing the wind to act on all airfoils simultaneously to produce the airfoil positions independently. Essentially, the system as a whole acts as the orienting mechanism for the individual airfoils. It accomplishes wind-direction sensing and airfoil-pitch control without external orientation mechanisms.
The VAWT described in U.S. Pat. No. 6,320,273 (i.e., the Nemec patent) illustrates an existing method of using an external device to position a pitch control mechanism in relation to the wind. Typically, an offset crank is oriented upwind or downwind of the principal axis of rotation and the individual airfoils set their angle-of-attack based on cyclical positioning relative to the hub and crank assembly. The Nemec patent emphasizes that in order to obtain optimal lift at all times, it is necessary to maintain a constant angle-of-attack of from four to ten degrees as the relative wind varies with rotor speed.
The helicopter described in U.S. Pat. No. 2,481,750 (i.e., the Hiller patent) uses a “Rotormatic” control system such that the tilt angle of the swashplate is controlled directly by the pilot's cyclic input. The swashplate in turn controls the flybar, a system of two lightweight symmetrical airfoils attached to a teetering bar oriented perpendicularly to the axis of the main rotor blades, and the flybar in turn controls the pitch of the main rotor blades. On receiving pitch changes to its paddles, the flybar responds by attempting to fly the new plane established by the swashplate. Because the pitch of the main rotor blades is controlled by the flybar, response of the flybar to the new control orientation is delayed. Eventually, the main rotor and the flybar reach equilibrium in the new rotational plane established by the swashplate.
Thus, the prior art has progressed to the use of airfoils in lift-based VAWT designs, with U.S. Pat. Nos. 6,688,842 and 6,749,394 providing additional related information. Nevertheless, the design of a suitable mechanically-controlled-pitch VAWT for power-generating purposes remains elusive despite the potential significant advantages of such a VAWT, including: (i) VAWTs do not require alignment with the windstream as does an HAWT, (ii) VAWT drive train components can be located at ground level instead of being mounted higher above ground at HAWT rotor level, (iii) VAWTs can have aesthetically more pleasing appearances, and (iv) a VAWT with a mechanically-controlled-pitch arrangement could avoid the less-than-ideal angle-of-attack and self-starting inefficiencies of self-positioning and fixed-pitch VAWTs. For these and other reasons, there exists a need for VAWT improvements.